Changes in the route matcher (removal of path-to-regexp)

path-to-regexp is a widely used library for converting path strings into regular expressions. In the Vue.js ecosystem, it was once a core dependency of vue-router, used for dynamic route matching. With the release of vue-router@4, path-to-regexp was removed in favor of a lighter, more flexible internal implementation. This change brought performance optimizations and a more streamlined API design.

Why path-to-regexp Was Removed

path-to-regexp is powerful but comes with some pain points:

1. Large Bundle Size: The minified size of path-to-regexp is about 4KB, which can be optimized for modern frontend applications.
2. Redundant Features: vue-router only used part of path-to-regexp's functionality, such as path parsing and parameter extraction, but the entire library was included.
3. Maintenance Overhead: Relying on a third-party library means dealing with updates, compatibility issues, and potential security vulnerabilities.

vue-router@4 reduced dependencies and improved performance by implementing path-matching logic internally. For example, the new matcher no longer needs to generate full regular expressions but instead directly parses path patterns.

Implementation of the New Route Matcher

The route matcher in vue-router@4 is based on simpler string analysis and parameter extraction. Here’s a simplified example of how dynamic route parameters can be parsed manually:

function parsePath(path) {
  const segments = path.split('/');
  const params = [];
  const regex = segments
    .map(segment => {
      if (segment.startsWith(':')) {
        params.push(segment.slice(1));
        return '([^/]+)';
      }
      return segment;
    })
    .join('/');
  return {
    regex: new RegExp(`^${regex}$`),
    params,
  };
}

const { regex, params } = parsePath('/user/:id/posts/:postId');
console.log(regex); // /^\/user\/([^/]+)\/posts\/([^/]+)$/
console.log(params); // ['id', 'postId']

This implementation avoids the complexity of path-to-regexp while meeting the basic needs of dynamic routing.

Impact on Developers

For most developers, this change is transparent because vue-router@4 maintains high API compatibility. For example, the following route configuration works in both vue-router@3 and vue-router@4:

const routes = [
  { path: '/user/:id', component: User },
  { path: '/post/:slug', component: Post },
];

However, note the following details:

1. Regex Syntax Differences: path-to-regexp supports more complex regex patterns (e.g., /user/:id(\\d+)), which may not be supported by vue-router@4's default implementation. For such features, a custom function for the path property is required.
2. Performance Optimization: The new matcher is faster when repeatedly matching the same path because it avoids recompiling regular expressions.

Custom Implementation for Advanced Route Matching

If a project requires more complex route-matching logic, a function can be passed to the path property of routes. For example, implementing permission-based route matching:

const routes = [
  {
    path: (to) => {
      const user = authStore.currentUser;
      return user?.isAdmin ? '/admin/dashboard' : '/dashboard';
    },
    component: Dashboard,
  },
];

This approach is more flexible and intuitive than relying on path-to-regexp's regex patterns.

Migration Considerations

When migrating from vue-router@3 to vue-router@4, check the following scenarios:

1. Regex Constraints for Dynamic Route Parameters: If advanced syntax like /user/:id(\\d+) was used, replace it with route guards or in-component validation.
2. Nested Route Matching Rules: vue-router@4 optimizes nested route matching, ensuring parent and child route paths concatenate correctly.
3. Redirects and Aliases: alias and redirect behaviors remain largely consistent, but edge cases may differ.

Here’s a migration example:

// vue-router@3 (using path-to-regexp)
{
  path: '/user/:id(\\d+)',
  component: User,
}

// vue-router@4 (using custom validation)
{
  path: '/user/:id',
  component: User,
  beforeEnter: (to) => {
    if (!/^\d+$/.test(to.params.id)) {
      return { path: '/404' };
    }
  },
}

Performance Comparison and Benchmarks

In real-world projects, removing path-to-regexp improved route-matching initialization speed by about 20%. Here’s a benchmark pseudocode:

// Testing path-to-regexp matching speed
const pathToRegexp = require('path-to-regexp');
const regex = pathToRegexp('/user/:id/posts/:postId');
console.time('match');
regex.exec('/user/123/posts/456');
console.timeEnd('match'); // ~0.05ms

// Testing vue-router@4 matching speed
const { regex, params } = parsePath('/user/:id/posts/:postId');
console.time('match');
regex.exec('/user/123/posts/456');
console.timeEnd('match'); // ~0.02ms

While the difference per match is small, it accumulates into significant performance gains in large applications (e.g., 100+ routes).

Ecosystem Adaptation

This change in vue-router@4 also affected related ecosystem libraries. For example:

1. Route Generation Tools: Libraries like vite-plugin-pages needed updates to their internal route-generation logic.
2. Testing Tools: @vue/test-utils's route mocking had to adapt to the new matcher behavior.
3. SSR Frameworks: Nuxt.js adjusted its route-parsing logic when adapting to vue-router@4.

Here’s an example of a community plugin adapting to the new matcher:

// Old version (based on path-to-regexp)
function generateRoute(path) {
  const keys = [];
  const re = pathToRegexp(path, keys);
  return { re, keys };
}

// New version (based on vue-router@4's logic)
function generateRoute(path) {
  const segments = path.split('/');
  const params = [];
  const regex = segments.map(segment => /* simplified implementation */).join('/');
  return { regex: new RegExp(`^${regex}$`), params };
}